It is known that supported cobalt containing Fischer-Tropsch synthesis (FTS) catalysts can be prepared by means of impregnation of a cobalt salt onto a catalyst support coupled with drying of the impregnated support, followed by calcination of the resultant dried impregnated support to obtain a FTS catalyst precursor. The catalyst precursor is then reduced to obtain the FTS catalyst comprising cobalt crystallites dispersed on the support.
It is also known that the manner in which the calcination step is carried out may influence the eventual activity of the catalyst. For example, WO 2010/011332 discloses a method of preparing a supported cobalt containing catalyst with homogeneously dispersed small crystallites. The method comprises depositing cobalt nitrate on a catalyst support and then heating the support in an oxygen containing, substantially water-free, atmosphere to about 160° C. to form an intermediate decomposition product. This intermediate decomposition product is then calcined and reduced to produce the catalyst with homogeneously dispersed small crystallites.
It is known (for example, from WO 2010/011332) that the activity of a cobalt containing FTS catalyst is proportional to the cobalt particle sizes above 6 nm, and accordingly it is expected that small crystallites and high dispersion thereof will result in improved activity of the catalyst.
U.S. Pat. No. 6,806,226 discloses a cobalt containing catalyst which, during the calcination step, has been calcined in such a manner that all reducible cobalt is of the formula-unit CoOaHb. It was found that catalysts so prepared had an improved activity.
It has now been found that if, in the preparation of a cobalt containing catalyst, the calcination is carried out at a space velocity above a certain value and a heating rate below a certain value according to the present invention, a catalyst with small crystallites is obtained.